1. Field of the Invention
The present invention relates generally to compositions useful for energy conversion and storage. More particularly, it concerns a method of preparing a composite cathode suitable for use in lithium batteries. The composite cathode includes nanometer size particles of a lithium spinel oxide and a sodium manganese oxide.
2. Description of Related Art
Commercially available rechargeable lithium batteries often use lithium cobalt oxide as the cathode (Nagaura and Tozawa, 1990; Scrosati, 1992). But cobalt is expensive and relatively toxic. The high cost and toxicity of cobalt-based cathodes have created tremendous interest in the development of manganese-based cathodes, because manganese is much less expensive than cobalt and environmentally benign. In this regard, the spinel lithium manganese oxide LiMn.sub.2 O.sub.4 has been investigated extensively over the years (Thackeray et al., 1983; Tarascon et al., 1994).
LiMn.sub.2 O.sub.4 shows two plateaus in voltage versus capacity plots, one at around 4 V and the other at around 3 V. While the 4 V region generally shows good cyclability with a capacity of &lt;120 mAh/g, the 3 V region exhibits drastic capacity fading upon cycling due to the macroscopic volume change associated with a cooperative Jahn-Teller distortion. As a result, the capacity in the 3 V region of the stoichiometric LiMn.sub.2 O.sub.4 spinel (about 150 mAh/g, theoretically) cannot be utilized. More recently, attention has been focused on the synthesis of layered LiMnO.sub.2, but this material shows poor cyclability due to the transformation of the layer structure to the spinel structure upon prolonged cycling (Armstrong and Bruce, 1996; Vintins and West, 1997).